With reference to FIG. 1, a ducted fan gas turbine engine which may incorporate the invention is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through and drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
The engine may have one or more seals installed, for example, between an interconnecting shaft and a casing for the shaft. Such seals may be so-called leaf seals.
Generally, leaf seals are used to form a seal between two relatively rotating components in order to provide a pressure barrier which defines high and low pressure areas. In the case of a gas turbine engine this helps restrict the leakage of air or fluid from particular areas of the engine. The pressure barrier is provided with a large number of typically rectangular leaves which are held at a defined angle to the radial around the seal circumference. The leaves are flexible and allow radial compliance which can accommodate changes in the radial position of the two relatively rotating components. The leaves are packed at a sufficient density to provide an effective pressure barrier but the use of leaves inevitably leads to interleaf gaps and porosity across the seal.
FIG. 2 shows a schematic perspective cut-away view of a portion of a typical leaf seal 210 comprising a pack of leaves 212. FIG. 3a shows an end view of a segment of a leaf seal 210 viewed along the axis of rotation of a rotating shaft 222. FIG. 3b shows a face view of a single leaf 212.
Each leaf 212 is in the form of a plate, each having a root end 214, a free end 216, axial width, w, and a thickness, t. The leaves 212 alternate with spacer elements 226 at the root end 214 and are secured to a backing ring 219 of a housing 218, which typically also comprises front 220 (high pressure side) and rear 221 (low pressure side) rigid annular cover plates. The free ends 216 of the leaves 212 present end edges 216a towards a surface of a rotating component 222 (shaft) generally rotating in the direction depicted by arrowhead 224. The leaves 212, and in particular the free end edges 216 of the leaves 212, act against the surface in order to create a seal across the assembly. Each leaf 212 is sufficiently compliant in order to radially adjust with rotation of the surface 222, so that a good sealing effect is created and maintained during use. The spacers 226 ensure that flexibility is available to appropriately present the leaves 212 towards the surface which, as illustrated, is generally with an inclined angle between them. The spacers 226 also help to form interleaf gaps 228 between adjacent working portions of the leaves 212. An axial leakage flow through these gaps 228 is induced by the pressure differential across the seal 210.
Generally, leaf seals can be designed such that the expected axial leakage flow determines the extent of the contact between the leaf elements and the rotating component. Thus, the leakage flow can contribute to leaf blow-down where the free end edges are urged radially inwards so as to bear on the rotor surface. Or blow-up forces which act to lift the leaf elements, thereby reducing the contact pressure but increasing the leakage flow. A limited amount of blow-down can be the more desirable to create a good seal between the free end edges and the surface, but excessive blow-down causes excessive rotor loading and wear in the seal and rotor. The wear of the end edges and/or the rotor can limit the usable life of the seal.
Various configurations of leaf seal have been proposed to help control the amount of blow-down. One example of this is described in WO06016098 which implements a leaf with an edge chamfer and an associated feature on the opposing cover plate, the separation of the two defines a control gap which is dependent on the radial deflection of a leaf.
A further complication in the design of leaf seals is brought about by the variance in the operating conditions. This variance is determined by many factors including but not limited by thermal expansion (differential and single bodied), operating pressures, mechanical tolerances and vibration. One particular issue of concern to this invention are variables which lead to a misalignment between the two components which are bridged by the seal.
The present invention seeks to provide an improved leaf seal which addresses some of the issues presented by having a misalignment.